1. Field of the Invention
This invention relates to a method and an apparatus for receiving light with a modulated polarization being propagated through an optical fiber.
2. Prior Art
Direct modulation is a technique for modulating light waves that has popularly been used for optical telecommunications. With direct modulation, the electric current being applied to a light source, which is typically a semiconductor laser device or a light emitting device, is modified in a controlled manner to modulate the output of the light source. However, indirect modulation, with which light (coming from a DC light source and) being propagated through an optical fiber is indirectly modulated by externally applying electric signals to the light by means of an optical external modulator, is increasingly gaining popularity in recent years.
There has been proposed a type of optical external modulator that utilizes acoustic optical effects. The (a) in FIG. 8 illustrates such an optical external modulator comprising an about 1 mm thick quartz glass substrate 41, an optical fiber 1 arranged on a surface of the substrate 41 and rigidly bonded thereto by means of an adhesive agent 43 of burnt granulous quartz glass and a piezoelectric device 49 arranged on the surface of the substrate 41 opposite to that of the optical fiber 1 and formed by sequentially laying a lower electrode 49, a piezoelectric film 46 and an upper electrode 47 to produce a multilayer structure as shown in (b) of FIG. 8.
When a high-frequency signal is applied between the lower electrode 49 and the upper electrode 47 of the piezoelectric device 49, the piezoelectric film 46 is driven to vibrate and generate a supersonic sound wave, which is then fed to the optical fiber 1 to locally modify the refractive index of the inside of the optical fiber 1 so that consequently the polarization of the light being propagated through the optical fiber 1 is modulated as a function of the applied high frequency signal.
The light passing through the optical fiber 1 can be received by means of an optical analyzer 29 comprised in a light receiving system and converted to light whose intensity is modulated as illustrated in FIG. 9. The conversion from light with a modulated polarization to light with a modulated intensity takes place with an efficiency that is highly dependent on the state of polarization of the light that strikes the optical analyzer 29 and therefore the angular position of the polarizer 25 and that of the optical analyzer 29 need to be rigorously controlled for optimization.
Methods have been proposed for effectively and efficiently converting light with a modulated polarization into light with a modulated intensity without modifying the angular position of the optical analyzer. Japanese Patent Application Laid-Open Publication No. Hei 3-206413 discloses such a method. With the disclosed method, the light transmission path is branched by a fusion-type optical waveguide coupler 70 so that the light branched by the coupler 70 is received by a pair of optical analyzers 29, 29 that are arranged at the respective output ports of the coupler 70 as illustrated in (a) of FIG. 10. The light transmission path can be divided into three or more than three branches by arranging two or more than two couplers 70 as illustrated in (b) of FIG. 10. A quarter-wavelength plate 72 may be inserted between one of the output ports of the coupler 70 and the corresponding optical analyzer 29 in order to give rise to a phase-difference bias as shown in (c) of FIG. 10. With any of the above arrangements, the signal transmitted through the path may be located at one of the ports with a high probability.
Problems to be Solved by the Invention
The above described known methods of modulating the polarization light is, however, accompanied by the following problems.
(1) The method of modulating the polarization of light as described above by referring to FIG. 9, where the angular position of the optical analyzer 29 is optimized, requires the modulation output to be fed back to the optical analyzer 29. A modulation system incorporating such a feed-back scheme would inevitably be large and practically not feasible.
(2) The method of modulating the polarization of light as described above by referring to FIGS. 10(a), 10(b) and 10(c) have proved to be unsatisfactory because the angular positions of the optical analyzers and the quarterwavelength plate 72 are not defined in the document. This will be discussed below.
Assume a plane perpendicular to the axis of light being propagated through an optical fiber and two components of polarization that are contained within the plane axed independent from each other. If an ultrasonic wave is being propagated along a y-axis and the amplitudes of the light wave along x-and y-axes are Ex and Ey respectively, they are expressed by respective equations as shown below. EQU Ex=E.sub.1 cos .theta..times.exp j(.omega.t-.beta.xZ) (1) and EQU Ey=E.sub.1 sin .theta..times.exp j(.omega.t-.beta.yZ+.psi.) (2),
where E.sub.1 is the electric field of the incident polarized light wave and is the angle formed by the polarized light (linearly polarized light) and the x-axis when the phase difference .omega. between Ex and Ey is equal to 0.
Equation (3) below is obtained by eliminating the time-dependent terms from the equations (1) and (2) above. ##EQU1## where .psi.+(.beta..sub.x -.beta..sub.y).sub.z is expressed by .psi..+-..DELTA..psi., .+-. being the variation in the polarized light.
Electric field E of the polarized light received by the optical analyzer is expressed by equation (4) below, where is the angle between the operating direction of the optical analyzer and the x-axis. EQU E=E.sub.1 {sin.sup.2 .theta..times.sin.sup.2 +sin 2.theta..times.sin .phi..times.cos .phi.-cos (.psi..+-..DELTA..psi.)+cos.sup.2 .theta..times.cos.sup.2 .psi.}.sup.0.5 ( 4)
The modulation output of the above system is given by the difference between the value of electric field E.sub.2 for .psi.+.DELTA..psi. and that of electric field E.sub.1 for .psi.-.DELTA..psi.. In view of the fact that the O/E converter is a square law detector. The modulation output V is expressed by equation (5) below. EQU V=sin 2.theta..times.sin 2.phi..times.sin .psi..times.sin .DELTA..psi.(5)
From the equation (5) above, it is understood that the optical analyzer is not effective for the detection of light with a modulated polarization when its operating direction is found in any of .theta.=0.degree., 90.degree., 180.degree., . . . or the direction along which an ultrasonic wave is propagated or a direction perpendicular to that direction, nor the analyzer is effective for detecting with a modulated polarization, if the modulation is focused on linear polarization (.psi.=0). Note that the modulation output of the above system is equal to 0 when .theta.=0.degree., 90.degree., 180.degree. . . . , or when linear polarization parallel or vertical to the axis of propagation of the ultrasonic wave is involved.
While the propagated light is branched and received by a plurality of optical analyzers 29 in (a) and (b) of FIG. 10, the operating directions of all the optical analyzers 29 can be found in any of .theta.=0.degree., 90.degree., 180.degree., . . . if the optical analyzers 29 are not rigorously regulated for angular position. On the other hand, the arrangement of (c) of FIG. 10 is incomplete because it uses a single optical analyzer 29. Additionally, with the arrangements of (a) and (b) of FIG. 10, the output would be 0 if light with a modulated polarization having an invariable axis of polarization and a mode of modulation focused on liner polarization is introduced to the system.
From the above discussion, it is clear now that any of the arrangements of FIGS. 10(a), 10(b) and 10(c) are not satisfactory for resolving the problem of undetectable state of polarization (hereinafter referred to a null point).
In view of the above problems, it is therefore an object of the present invention to provide a method and an apparatus for receiving light with a modulated polarization that can detect any modulation output of an optical external modulator by means of optical analyzers that are rigidly held in position.